Geophysical exploring



Oct. 4, 1960 J. LA Mo'r-rE sHAw ETAL 2,955,251

GEoPHysIcAL ExPLoRING Filed March 21, 195e 4 sheets-sheet 1 Oct. 4, 1960J. LA MOTTE sHAw ETAL 2,955,251

GEoPHYsIcAr. ExPLoRING Filed March 21. 1958 4 Sheets-Sheet 2 BY@ @fmOct. 4, 1960 J. LA MOTTE sHAw Erm. 2,955,251

GEOPHYSICAL EXPLORING 4 Sheets-Sheet. 3

Filed March 2l, 1958 .50o (aPPHox.)

/fm 34 comm/MMG mms/wrm? @a fiers/rsa) co/s Oct. 4, 1960 J. LA MOTTEsHAw ErAL 2,955,251

GEOPHYSICAL EXPLORING Filed March 2l, 1958 4 Sheets-Sheet 4 UnitedStates Patent GEOPHYSICAL 'EXPLORING John La Motte Shaw and Ronald RexTaylor, Copper ClitLVOntario, Canada, assignors to The InternationalNickel Company, Inc., NewV York, N.Y., a corpora- `tion of DelawareFiled Mar. 21, 1958, Ser. No. 722,954 Claims priority, applicationCanada Oct. 24, 1957 3 1 (CI. 324-4) 'The Mesem .invention relates t0.methods and apparatus for electrically detecting and measuring mineralairborne operations and, more particularly, to an, improved,`electromagnetic method and to improved electrical equipment adapted forthe commercial applica- `detection and measurement of the mineralbodies.

is well, known to those skilled in the art, various electromagneticmethods have. been employed in detecting and measuring mineral bodies byairborne operations. Qertain of these prior systems and methods employ astreamlined, bomb-shaped case, called a bird or ibonrbg towed by anaircraft, which bird contains the receiver apparatus while thecoperating transmitter apparatus is located within the aircraft.However, substantial diiculties have been encountered due to thevariations in electrical signals, which variations are at- ICC oneairbornecraft and the receiver is in a second airborne craft togetherwith special means to maintain `a substantially constant positionalrelationship between'the two craft.

The yinvention further contemplates providingian. fimprovedelectromagneticmethod of ore prospecting.

Still #another object of the invention is to provide` an improved methodfor achieving accurate airborne electromagnetic measurements of thelocations of mineral bodies.

It is also an object of this invention to provide an improved method forairborne electromagnetic prospecting in which corrections arey made forpositional variations between the transmitting and receiving coils.

Other objectssand advantages Willbecome. V' .1llpare'nt from thefollowing description taken in conjunction with the accompanyingdrawing, in which:

Figure 1 is a diagrammatical view of oneillustretive embodiment of thisinvention;

Figs. la to 1d, inclusive, are diagrammatical viewsde.- pictingillustrative embodiments of the general system shown in Fig- 1- Fig. 2.shows in schematic and block forman embodiment of an electromagneticapparatus employed in the general system depicted in Fig. l; Y

Fig. 3 depicts a diagrammatical view of another illus,- trativeembodiment of this invention;

Fig. 4 illustrates in schematic and block form an embodiment of theelectromagnetic apparatus that maybe employed in the general systemdepicted in Fig. 3` when the transmitting coils are carried by the birdor bomb;

Fig. 5 depicts in schematicand block form an fembodiment of theelectromagnetic apparatusV that may .be employed with the general systemshown in Fig. 3 .when the receiving coils are carried in thebird orbomb;

Fig. 6 depicts in schematic and block form an electrooptical system thatmay be employed inY accordance with this invention and depicts thelight-.responsive apparatus,

tributable to the movement of the bird relative to the air- A craft. Forexample, a bird towed by an aircraft undergOSS certain motion relativeto the aircraft due to various disturbances such as turbulence andvariations in the speed of the aircraft. This motion of the birdrelative tothe aircraft changes the relative position of the receivingcoils tof the transmitting coils which may result in responses fromother than conductive bodies confusinglltsimilar to the type ofresponses that would be expected from conductive bodies.

It has now -been discovered that the foregoing -dif- Viietilties may beobviated by employing certain special apgangements of the transmittingand receiving coilstogether with special means to maintain asubstantially constant positional relationship between the transmittingand receiving coils.

It; is an object ofthe present invention to provide improved apparatusfor electrically detecting and measuring mineral lbodies by airborneoperations.

It is another object of this invention to achieve accurate airborneelectromagnetic measurements of the locations of mineral ore bodies byemploying special means to eliminate positional errors-developed withinthe ore detection and measuring system.

It is a further object of this invention to provide a spec ial systemfor correction of positional variations between the transmit-ting andreceiving coils in an airborne electromagnetic prospecting system.

.It is still another object of this invention to provide an airborneelectromagnetic prospecting apparatus in including a vertically-opposedpair of ylightfsensitive, cells,

Yas viewed on line 6-.6 of Fig.. S;

Fig. 7 illustnates the lighbresponsiver viewed from line. 'l-.7 of Fig.8;

Fig. 8 is a View of the optical pyramid and ,cluster of photo cellstaken on line A-A of Fig. 6,; and

Fig. 9 depicts the. electrical connections of Atllepositional controlportion of the electro-opticalsystem.of

apparatus as Figs. 6 and 7.

Generally speaking, the present invention contemplates an airborneelectromagnetic prospecting vmethod and apparatus `therefor in which theelectromagnetic fieldgenerating coil or coils are carried by oneairborne craft and the receiving coil or coils are carried byanotherairborne craft. The/two airborne coil-carrying crafts may `be movedthrough the air in any suitable manner.

Advantageously, these coil-carrying craft may be birds or bombs (ashereinbefore defined), each .trailed by a However, one of thecoilcarrying craft may be a powered aircraft with the other constitutinga bird or bomb towed on a cable from the powered aircraft. Preferably, adual-.frequency method of airborne prospecting is used which comprisesrradiating electromagnetic fields from a pair of mutually orthogonaltransmitting coils and receiving the responses with a pair of mutuallyorthogonal receiving coils in which one Vwhich the electromagnetic fieldtransnutting means ,isf-in transmitting coil and one receiving coil arecoplana-r and the other pair are coaxial. Although the dual frequency.method does compensate for limited relativewmovements between thetransmitting and receiving coils, ithas: been found that deviationsbeyond these limited relative movements do occur suliciently often torestrict surveys to fairly smooth dying 4conditions and may adverselyaffect .the accuracy ofltheeresults even under satisfactory; Y,

conditions. Since the birds are inherently less stable than the heavierpowered aircraft, the double trailing system, i.e., with thetransmitting coilsY in one bird and the receiving coils in a separatebird, makes the occurrence of these larger deviations all the morelikely. Preferably, a special independent error or positional controlsystem is provided to bring these movements within satisfactory limitsand to maintain Vreceiver-transmitter alignment.

Referring now to Fig. l, there is depicted an aircraft towing a rst bird12 by means of a relatively short tow cable 14 and also towing a secondbird 16 by means of a relatively long tow cable 18. The first or leadingbird 12 may advantageously contain the transmitting coil or coils forgenerating the electromagnetic iields. The iirst bird 12 is towed at ashort distance from the aircraft, for example, about 75 feet, so thetransmitting coil or coils in the first bird may radiate one or moreelectromagnetic iields which will be substantially unaifected by themetallic structure ofthe aircraft. The relatively heavy transmitters orpower sources `for energizing the transmitting coil or coils may becarried in the aircraft to minimize the weight of the transmittingcoil-carrying bird and thus reduce the size of tow cable '14 required tosuspend the bird. The length of cable 14 is also maintained relativelyshort to minimize the transmissionlosses in the electrical conductorconnecting the transmitters or power sources and the transmitting coilor coils, which conductor is located in cable 14. The receiving coil orY' 'coils ymay be located in the second bird to receive the variationsin the generated electromagnetic fields due to the presence of variousbodies in the earth. Although for the reasons pointed out hereinbeforeit is advantageous to have theY transmitting coils in the iirst orleading bird 12, if desired the receiving coils may be carried in theleading birdlz and the transmitting coils carried in the second ortrailing bird 16. Y

Referring now to Pig. 2, there is depicted in block form, anelectromagnetic system that may beV employed in conjunction with thetwo-bird system as shown generally in Fig, l. The aircraft may containthe transmitters or power sources for energizing the transmitting orelectro magnetic field radiating Vcoil orV coils. coil or coils may beVcarried in a bird towed by the aircraft. As illustrateddiagrammatically in Fig. 2, the

aircraft is indicated generally by block 10 and it may contain a pair oftransmitters or power sources T-i and T-2 for energizing a pair of4transmitting coils 32 and 33 individually connected by conductors` 30and 31 extending through tow cable 14 to Vtransmitting coils 32 and 33,respectively. The transmitting coils 32 and 33, as shown in Fig. 2, maybe orthogonally positioned with respect to each other in the first bird12. The receiving cilor coils may be carried in a second bird, indicatedgenerally by block 16, and may include a pair of orthogonally positionedreceiving coils 37Vand 3S. Receiving coil 37 maybe positioned incoplanar relationship with the transmitting coil 32. The receiving coil38 and its corresponding transmitting coilv 33 may be oriented coaxialwith each other'.Y AThe Vtransmitting and receiving coils are thusarranged in relationto each other so errors dueto rela- Y f tive motionbetween them are minimized. Advantageously,

one set of transmitting and receiving coils 32 and 37 operate at afrequency f-lV and another pair of transmitting and receiving coils 33and 38 operate at a different fre- Yquency f-2. The frequencies used arewithin the range of near zerorto about 29,000 cycles per second and areselected from within this range to give the best compromise betweenweight and performance. The factors that are applied in a general wayare listed below:

(l) A Vlow frequency is preferable lin prospecting for good sulfideconductors.

(2) The lower the frequency, the larger and heavier is the equipmentwhich must be carried.

(3) Separation of the two frequencies musty be suffi- .cient for thesatisfactory filteringV of one from the other.

'Ihe transmitting Y,

and may adversely yaiect the accuracy (4) One frequency is preferablynot a harmonic of the other.

(5) When employing a helicopter, rotor modulation should be considered.For example, a fourbladed rotor turning at 600 r.p.m. would have a rotormodulation of 40 cycles per second (cps.) (4X600/ 60). Accordingly, thefrequencies used would be selected from within the foregoing range tominimizepany elfect of this rotor modulation. v

(6) Mechanical resonances may be avoided by design of the structure tostay out of detecting frequency range.

A first amplier A1 visV shown connected'to receiving coil 37 anda secondamplifier A-2 is shown connected to receiving coil 38. The output ofamplifiers A-'1 and A-2 may be impressed on an electrical carrier wavein a modulator i4 to facilitate transmission of the signal along the towcable to the aircraft. -In the amplifiers A1 and A-Z the signals fromcoils 37 and 38 due to each of the electromagnetic fields including Vtheresponses due to Vearth formations are applied at a level high enough tooperate the modulator 44. vIt is also desirable to have A1 and A-2 asacombined amplifier amplifying both signals simultaneously so thatvariations in gain are canceled. The modulated electrical carrier waveoutput of modulator 44 is transmitted by the electrical conductor 36 intow cable 1S to a detecting and Vcomparing circuit 46, preferablylocated within the aircraft 19. In the detecting and comparing circuit46, the signals from amplifiers A-l and A-2 are detected and separatedfrom the electrical carrier wave and compared. The output of thedetecting and comparing circuitV `46 may be, if desired,V connected to asuitable recorderriii,l also preferably located in the aircraft 10.

As .pointed out hereinbefore, although the aforementioned Vdualfrequency method of electromagnetic airborne exploration does compensateVfor some relative movements between the transmitting coils and thereeeivirigv coils, it `has been found that deviations beyond suchlimited relative movements do occur sufliciently often to restrictsurveys to fairly smooth flying conditions of the results even undersatisfactory iiying conditions. The occurrence of these largerdeviationsV are all the more likely to occur in the double trailingsystem fas depictedV in Figs. l and 2 wherein thetransmitting coilsV arein one bird and the receiving coils arevin a separate bird. It has beenfound desirable `to provide `a special error or positional controlsystem, independent of the foregoing electromagnetic exploration systemexemplified by Fig. 2,Y to bring these 'relative movements ofthecoil-carrying craft within satisfactory limitsand generally to maintainreceiver-transmitter alignment. Preferabli'the error or positionalcontrol system comprises a'special electro-optical system as will beexplained in detail hereinafter.

The most preferred electro-optical system for the error correction orpositional'control apparatus for the double trailing'airborneexploration system is depicted generally in iFig.` l. A source Vof light,27 is located at a convenient position in or on the aircraft 1t). Areceiving section 28 of the electro-optical system is located preferablyin the first bird 12.l The Vlight source 27 is arranged to direct a beamof light from the aircraft 10 at the light receiving section28 in thefirst bird 12. Advantageously, the source of light 27 includes means formodulating lthe light beam, and the light receiving section of theelectro-optical system is constructed to distinguish between themodulated light from source 27 and other light from other sources, suchas sunlight or reflections. The light receiving section 28 of theelectro-optical system which is located in bird l2 may be connectedthrough suitable mechanical and electrical means to the control iins 19of this bird in such a manner as to actuate control ltins 19 to maintaina vrelatively constant positional mittingcoil'and the powered aircraft10. The light sensi- :ive devices of the section. as of the@ecumenical-system, which may advantageously be photo cells or photomultipliers, are preferably located in the forward end of the lirst bird12 and may advantageously be located; on the upper surface of theforward end of the bird. A second and Similar electro-optical system isused to. control the bird 1,6 containing the receiving coils from thetransmitting coil-containing bird l2. A second light source 3.9 islocated in the first bird l2 and a second light receiving section 4t) islocated in the second bird 16. The Second light source 39 is arranged inbird 12 to direct a light beam, which advantageously may be a modulatedlight beam, at thev second light receiving section 4 0 in second bird1,6 containing the receiving coils. The second light receiving sectioniti in bird. l5 is connected through suitable. mechanical and.electrical means to the control ns 19 of this bird in such a manner asto actuate these control ns l19 to maintain a relatively constantpositional relationship between the rst and second birds and hencebetween the transmitting coils 32 and 33 and their respective receivingcoils 37 and 38.r =It should be understood that where a high degree ofcontrol is not required, it may be suicient to controlthe bird locontaining the receiving coils from a light source in the bird 12containing the transmitting coils without -controlling the bird 12containing the transmitting coils from a light source in the yaircraftlil as depicted in Fig. la. However, under .turbulent `air conditionsthe variations of the bird l12. containing the transmitting coils mightbe excessive. and this might put a severe strain on theV ability of thebird. 16 containing the receiving coils to. followY the oscillationsrofthe bird l2 containing the transmitting coils.

Another preferred 'arrangement of the electro-optical error correctionor positional control system is shown; in Fig, lb.- rlhis, arrangementinvolves,A providing two light sources 27 and 27', of the typepreviously described, suitably mounted in the powered aircraft 10 witheach light source, arranged to direct a modulated light beam at aseparate one of the two light receiving sections 28 and 40 located,respectively, in the transmitting coil-containing bird 12; and in thereceiving coil-containing bird 16.

In a further desirable embodiment of the electro-optical errorcorrection or positional control system illustrated in Fig. lc, thesingle light source 27- located at the aircraft 1,6 is. utilized todirect separate beams of modulated light at the light receiving section2d in the bird 12 and/at the light receiving section 4,0 in the bird 16.Thus, each of the birds is separately maintained in relatively constantpositional relationship between the aircraft, l and the bird.

A further suitable embodiment of the electro-optical error correction orpositional control system `includes both control of the receiving coilalignment with respect to the transmitting coils and transmitting coilalignment with respect tothe receiving coils as, shown in Fig. la'. Inthis embodiment a light source 39 in the transmitting coilcontainingbird 12 directs a modulated light beam at a light receiving section 46.of the electro-optical system located in the receiving coil-containingbird 16, to control the positional relationship of bird 16 with respectto bird 12. A second light source 41 is arranged` in the second bird 16tQ direct a modulated light beam at a second light receiving section 42in the rst bird. 12 t0 provide additional control of the positionalrelationship of the transmitting; coil-containing bird 12,y with`respect to the receiving coil-containing bird 16. It is` understood thatwhere complete control is not. required,y one of these systems can beused alone, for example, 3S. depicted in Fig.A la.

Fig. 3 depicts a further illustrative embodiment of the invention inwhich an aircraft Ztl has a housing or extension 2.2 made ofelectrically non-conductive material,A such as a plastic material,located, for example, below the tail section of the aircraft. Thishousing may contain the receiving Coil 0r Coils or, if desired, thetrans.

-A`:hitting cell. ercoils.- .Bird 2.4 ltowed by aircraft 2.a b. Ineas.ef. Mela vely lone Gable 26- The transmitting coils, ory the receivingcoils as the case may be, are locatedin bird 24. Advantageously, a lightsource Z'Z may be suitably located on the aircraft 2t?. and arranged to'direct a modulated light beam ata light receiving section 2&ofanelectro-optical control system located in bird 24. As in the previouslydescribed embodiments, the light receiving section 28 in `bird 24 is,connected through mechanical and electrical means to the control hns.Z9. of this bird in sucha manner as. to actuate the control hns 2910maintaina relatively constant. positional relationship between the bird24 and theV aircraft .29. .to assure. ac.- curate. measurements of the.presencer of ore. bodies. .Of course, if desired, the light Vsource 2.7may be. suitably positioned in the bird 2e and kthe light receivingsection 3 of theA electro-optical control system may. be suitablyy 1.o.-cated on the .aircraft 2.0..,

vReferring now to Fig. 4,. there is. depicted in schematic and blockform an. arrangement of the electromagnetic ore detecting andmeasurinalapparat11s which may be. ernployed in the. single bird .systemw showngenerally in. Fig. 3. ln, this arrangement, the aircraft, the plastichousing andthe. birdof Fig` 3 areindicated generally in Fig. 4.125;blocks 20,22 and 2.4 respectively. The aircarft 210 may contain the,transmitters. or power soluces for. 611ergizing., the .transnttina ereleetromaenetie eld radiating, coil orteils located. inthe bird.. The.corresponding te'- ceiving: coil orcoils are suitablvpositioned in. er,0.11 the aircraft.. As indicatedv in Fie. 4,. the aimait@- rtly' @DI1-tan. a pair @transmitters ornower Sources, Tf1` andV "1T-,2 which. areconnected through ConluClOrSSl and 5.3 111.1 tow cable. 2.6. to the.transmlttnaeols 332 and 3S, testeo tively, in. the bird. 2t t0. radiateelectromasnetieflelds at difterentfrequencles f-lV and ft ln thisarrangement, the receiving c of1'1s37' and. -.are located. Within the.1.19118.- ingl 22 at the tail section of the aircraft 201 and areelectrically Connected to. a pair Qf empllflers Afl; @11d-Av2., whichare in turn connected to. a detecting and comparing circuit .467 in theVaircraft. A, recorderr 48, may be Connected. to. the output 0f thedetecting and Comparing circuit 4t. The transmitting and receivingcoils, are related to each other in the same manner as in the embodimentof' Fig 2. The electrical circuits of the mineral detecting andmeasuring apparatus are the same as Iin Fig.`2, except that themodulator has been omitted. Of course, a modulator may be includedY asvin Fig. 2' ifdesired. The frequencies are selected, andtheairbornefelectrornagnetic ore detecting and measuring operations areconducted,I in the samemanner as explained for Fig. 2:.

rReferring now to Fig. 5;, there is depicted in schematic and blocklform another erterrlllary embodiment of the electromagnetic oredetecting and measuring systemof this invention as employed in thegeneral arrangement shown in Fig.,` 3. rn'Fig;V 5 the aircraft 2li, thehousing 22 `andy the bird 24' are indicated generally *byr blocks-2li,22 and 24.v The aircraft20 mhycontain the transmitters or power sourcesfor energi'zingfthe transmitting, or electromagnetic el'dradiating,rcoil or coils suitably positioned in or onv the, aircraft.The corresponding receiving coil or coils are inthe bird As indicatedinl Fifg-.-5', the aircraft 2.0 may contain a vPair of transmitters orpower sources T-l and T-2 which are connected to and energize. thetransmitting vcoils 32 and 33, respectively, located in the plastichousing V22, at the tai-l section ofV the aircraft to radiate Yelectromagnetic -iields at different frequencies )L1 and f-Z. Receiving`coils137 and 38. are located in the bird-24 which also containsampliliers A-1 and A-21 connected to the receiving coils 37 and'38,respectively. The output leads of the ampliers Aal and A-2 are connectedto. a modulator 4'4which is.4 also located within the bird. The outputof modulator 44 is connected, through a conductor- 45 contained withintow cable 26 to a detecting `and comparingcircuit. 46 located within theaircraft20. AThe output of'thedetecting j systems, the preferredembodiment of `provides a beam of modulated light. n

which passes through the circle of apertures in apertures in Vradialangles,

Verally in block 2S.

7 andV comparing circuit 46 Ymay be connected to a suitable recorder 43which also may be located in the aircraft 20. Thetransmttingrandrreceiving coils are related to each other, and the partsof the mineral detecting and measuring apparatus are electricallyconnected to each other,V

in the same manner as in the embodiment of Fig. 2. The frequencies areselected, and the airborne electromagnetic ore detecting and measuringoperations are conducted, in the same manner as explained hereinbeforefor Fig. 2.

As disclosed hereinbefore, the various embodiments of the airborneelectromagnetic exploration apparatus of this invention desirably employan error correction or positional control system to compensate forrelative movements between the transmitting and receiving coils and ingeneral to maintain receiver-transmitter alignment, and preferably theyuse therefor one or more electro-optical whichrhas been only generallydescribed hereinbefore. Figs. 6 through 9 are now referred to for a moredetailed description of the preferred electro-optical error correctiontransmittingreceiving coil positional alignment control system that maybe employed in the various embodiments of this invention. Fig. 6 depictsthe electro-optical system as including both a light source indicatedgenerally by block 27 and a light-receiving or electro-opticallight-responsive section indicated generally by block 2S. The lightsource or block 27 is located at the craft with respect to which theposition of another craft is to be controlled. The light-receivingsection 28 is located at the craft whose movements are to be controlled.The light source 27 This light beam is modulated in any suitable manner,for example, electrically or mechanically. Block 27 preferably includesa regulated power supply 58 connected to a lamp 60 to provide a sourceof light of constant intensity. A perforated disk 64 is connected to theshaft of a motor 62 which, if desired, may be connected to the powersupply 58. Y The perforated disk 64 is positioned transversely acrossthe path of the light from lamp 63. A suitable lens 66 is aligned in thepath of the light from lamp 60 disk 64. By driving the motor 62 atconstant speed and employing disk 64 Which are spaced apart Yat equalthe light which passes through lens 66 will be interrupted or modulatedat a constant rate.

The electro-optical light-responsive apparatus for receiving themodulated light from lens 66 is shown gen- Block 28 includes a suitablelens 68 to collect the modulated light and an opticallight-distributingdevice 70. The light-distributing device 70 is a four sided regularpyramid having its axis aligned with the optical axisV of lens 68. Thislight distributing device may be made of any suitable transparentmaterial, eg., optical glass, which may transmit light through its fourequal inclined faces. However, if desired, the four Vequal inclinedfaces of this pyramid may have a'mirror finish to reflect light fromVthe faces, in which case of course the optical device 70 need not bemade of a transparent material. This optical device 70 may be referredtothroughout this description and'in the claims as a pyramidal prism.Adjacent each side or face of the pyramidal prism 70 is a lightsensitive device such as a photoelectric cell or phototube.'Ylhotoelectric cells 72a: and 72b, for example, are shown in Figs. 6 and8 adjacent the upper and lower faces of the pyramidal prism 70.Photoelectric cells 72C and V72rd Yare shown in Figs. 7 and 8 adjacentthe other two faces ofthe pyramidal prism 70. A View of the pyramidalprism 79 and its cluster of surrounding photo cells or phototubcs 72a,72b,

Y 72C and 72d is Vshown in Fig. 8. If the beam of light is focused onapex 73 of the Apyrarnidal prism 70 so that equal light` will bereceived by each of the four photo i Acells 72a to-72al, Vthen theoutput-signal derived from each of theseV photoV cells willbe equal.This is the normal orientation of the light-receiving section 2S withrespect vto the light beam when the receiving coils such as 37V and 38are properly aligned in their preferred position with respect toVthe'transmitting coils such as 32 and 33. The outputl leadsV of eachpair of opposed photoelectric cells are connected to electric circuitsshown generally by block 74 in Figs. 6 and 7 and shown more in detail inFig. 9. Thus, the vertically-opposed pair of photoelectric cells 72a and72b are connected to suitable l0 electric circuits in the block 74 andthe output thereof is in turn connected to a control or orienting motor76 to operate horizontal control tins on the craft whose position is tobe regulated. The horizontally opposed pair Y of photoelectric cells 72Cand 72d are connected to suitable electric circuits in the block 74 andthe output thereof is in turn connected to a control or orienting motor78 to operate vertical control fins on the craft whose position is to beregulated. These motors 76 and 78 are provided with suitable connectionsto operate the control fins of the craft Whose position is to beregulated. Any

deviation from the normal orientation in which the transmitting andreceiving coils are in proper alignment causes an increase in the lightfalling on one of these cells and a decrease in the light falling on theopposite photo cell 2 which causes the appropriate motor 76 or '78 tomove control surfaces that return the bird to its preferred position.

Referring now to Fig. 9, there is depicted in block form the four photocells and the electrical control apparatus shown generally in block 28on Figs. 6 and 7. As depicted in Fig. 9, the electrical apparatus ofblock 74 includes four separate filter and amplifier circuits 80a, Stlb,Sile and 80d, one for each of the photo cells. These filter andYamplifier circuits each include a filter (not shown in detail) which istuned to the frequency of modulation of the light beam. The provision ofa modulated light source in combination with the filters tuned to thefrequency of modulation of the light source thus eliminates interferencefrom spurious light changessuch as sunlight, reflections, etc. Theseelectric circuits also include a first comparison and control circuit82a connected to theoutputs of the filter and amplifier of circuits 80aand 80h of the pair of opposed photo cells 72a and 72b. The output ofcircuit 32d is connected to the input of orienting or control motor 76,which motor in turn is connected in a suitable manner, e.g.,mechanically, to the horizontal control fins of the bird. The expressionhorizontal control fins when used in this description and in theappended claims is intended to signify the fins which regulate themovement of the craft in a vertical plane. A second comparison andcontrol circuit 82h is connected to the outputs of filter and amplifiercircuits 80e and Stiel of the other pair of opposed photo cells 72C and72d. The output of circuit S2b is connected to the input of orienting orcontrol motor 78. Orienting motor 7S is connected in a suitable manner,eg., mechanically, tothe vertical control fins of the craft. Theexpression Vvertical control fins when used in this description and Yinthe appended claims is .intended to signify the fins which control thedirection of the craft sideways or in `a horizontal plane.

In the operation of the electro-optical positional control apparatus inthe system shown generally in Fig. 3, a source 27 of modulated light islocated in Vthe aircraft 654 20. The beam of light is projected in thedirection of bird 2.4 so that the light will fall on the lens 68 of the`light-receiving section 28 of the electro-optical apparatus in bird 24.The axes of lens 68 and pyramidal prism 7G are aligned in the generaldirection of the aircraft and, 70 more particularly, in the direction ofthe beam of moduvlated light as indicated inFig. 6. When the bird ispositioned and oriented with respect to the aircraft 20 so that thetransmitting and receiving coils are properly aligned, `a spot-.of lightfrom the modulated light source 27 is projected on the apex 73 of thepyramidal prism '70 in 9 such apmanner that all four faces of thepyramidal; prism are equally illuminated. If, due to the turbulent airconditions or other causes, the bird moves or swings to the left of thecenter of the beam ofv light, photo cell 72d will receive less light andphoto cell 72C will receive more light, resulting in decreased andincreased signals, respectively, and vice versa if the bird moves orswings to the right which signals will be transmitted by circuits 80eand 80d to comparison and control circuit 8217. In accordance with thedirection of the unbalance, an appropn'ate control signal is deliveredto orienting motor 78 which causes the vertical control fins to turn thebird sideways lin the direction of the center of the beam of light.Similarly, if, due to turbulent air conditions or other causes, the bird24 were to rise or fall or swing vertically out of line with respect tovthe light beam, the light signal delivered to one of the photo cells72a and '72b would be decreased and the light signal delivered to theother of the two photo cells would be increased. These unbalancedsignals are delivered through circuits 80a and 80b, respectively, tocomparison and control circuit 82a. The resulting control signal isdelivered to orienting motor 76 which in turn causes the horizontalcontrol fins to return the bird to a position in which the of the lightsignal is again aligned with the axis of prism. Thus, theelectro-optical system maintains a relatively constant positionalrelationship between the transmitting and receiving coils and eliminatespositional errors which might otherwise deleteriously affect thedetection and measurement of the bodies. Y

Similarly, in the operation of the electro-optical error correction andpositional control system in the arrangement shown generally in Fig. l,the source 27 of modulated light may be located in the aircraft 10 and alight-receiving section of the electro-optical apparatus may be locatedin the nose of the iirst bird 12 so that the first bird 12 containingthe transmitting coils may be `maintained in substantially constantpositional relationship with respect to the aircraft 10. A second lightsource 39 is provided in the iirst bird 12 to project a modulated lightbeam on to a second light-receiving section 40 in the second bird 16which contains the receiving coils. Accordingly, this secondelectro-optical control apparatus-operates in the same manner tomaintain the receiving coils in the second bird 16 in proper alignmentwith the transmitting coils in the rst bird 12. As pointed outhereinbefore, where a high degree of control is not required,l it may besuicient to use the foregoing second set of light source andlight-receiving sections to control the bird 16 containing the receivingcoils from a light source in the bird 12 containing the transmittingcoils without controlling the bird 12 containing the trans-(- mittingcoils from a light source in the aircraft 10. It may be desirable in thearrangement shown generallyl in Fig'. 1 to direct two beams of lightfrom the aircraft either from a single source of modulated light or eachi beam from a separate source of modulated light in the aircraft. Asbefore, separate light-receiving sections 28 and 40 are located,respectively, in the first bird 12 and in the second bird 16. One ofthemodulated light beams from the aircraft is directed at thelight-receiving section in bird 12 and the other modulated light beam atthe light-receiving section in the bird 16. With this arrangement, farelatively constant positional relationship is maintained between eachof the birds and the aircraft, thereby providing a relatively constantpositional relationship between the two birds and also between thetransmitting coils and the receiving coils. As pointed out hereinbefore,it is also possible in the two-bird system depicted generally in Fig. lto utilize two electro-optical control systems such that a light sourceof one system is positioned in the first bird 12 and its light-receivingsection is located in the second bird 16 and the light source of thesecond electro-optical system is located in the second bird 16 with itslight-receiving section in the lirst l0 bird 12'. Thus, one of thesesystems maintains transmitting coil alignment with the receiving coilsand the other electro-optical system maintains receiving coil alignmentwith the transmitting coils. It is understood, of course, that wherecomplete control is vnot required one of these electro-optical systemscan be used alone.

Although the light source need not be modulated, it is desirable thatthe source of light be modulated Vand that the electro-optical apparatushave associated there.- with filters which permit the passage of onlymodulated signals in orderfor the system to be insensitive to all othersources of light such as sunlight 'and reections.

The electromagnetic mineral detecting equipment and its operation m-aybe similar to that disclosed inthe Cartier et al. U.S. Patent No.2,623,924, issued December 30, 1952. As described in that patent, thesystem may employ mutually orthogonal transmitting coils to radiate twofrequency-distinguished electromagnetic fields, and a second pair ofmutually orthogonally-.positioned coils to receive the responses fromthese coils due to .the presence of various conductive bodies, and inwhich the received signals are combined and ampliiied by a cornmonamplifier, then separated by lters into the two frequencies and the twofrequencies finally combined to indicate the conductive bodies.

It is to. be observed that the present inventionprovides electro-optical`apparatus to maintain the transmitting coils in a relatively constantpositional relationship with the receiving coils of an airborneelectromagnetic prospecting system and thus accurate detection andmeasurement for and location of ore bodies.

It is also to be observed that the present invention provides anelectro-optical error correction and positional control apparatus thatmay be used in the System of the aforementioned Cartier et al. U.S.patent tov maintain the transmitting coils in a relatively constantpositional relationship with the receiving coils of the airborneelectromagnetic prospecting system of that patent and thus ensureaccurate detection and measurement for their location of ore bodies.

Another improved system for electrically detecting and measuringconductive mineral ore bodies by airborne operation isy disclosed anddescribed in our application `Serial No. 723,004, led March 2l, 1958. Astherein disclosed, the electromagnetic coils are located in a singlebird suspended from a hovering type aircraft.

Still another improved system for electrically detecting andk measuringore bodies by airborne operation is disclosed and-described in ourapplication Serial No. 722,895, tiled March 2l, 1958. ln accordance withthel system therein described, three electromagnetic fields areradiated, detected and comparisons made to detect and measure.conductive ore, magnetic ore, and mixtures of such ores.

It is further to be observed that the invention provides improved methodand apparatus for controlling the positional relationship betweenairborne craft or vehicles, one of which may be a powered aircraft andone or more of which may comprise a bird or other vehicle towed byanother aircraft or otherwise moved through the air and wherein thedesired substantially constant positional relationship between theairborne craft or vehicles is attained by an Velectro-optical meanscarried by each of the airborne craft whose relative position is to becontrolled. It is also to be observed that the electro-optical means inone of said airborne craft may comprise a source of' light and means fordirecting a beam of light from the source in the direction of anotherairborne craft and the electrotrolled and the electro-optical'means ineach of the latter airborne cr-aft comprises electro-optical apparatusresponsive to a separate one of said light beams to maintain asubstantially constant positional relationship between said two airbornecraft. Y

It is to be observed that, while it is not essential to include amodulator, reduction of spurious signals, such as those that vmay bepicked up in, or caused by, the long tow cable, may be obtained byimpression of the received signals on an electrical carrier wave as inthe modulator 44 in the bird followed by transmission of the modulatedelectrical carrier wave to the detecting and comparing circuit 46wherein the signals due to the one or more electromagnetic elds,including the responses due to earth formations, are detected andseparated from theV carrier wave. Although the present invention hasbeen described in conjunction with preferred embodiments, it is to beunderstood that modifications and variations may be resorted to withoutdeparting from the spirit and scope of the invention as those skilled inthe art will readily understand. Such -modications and variations areconsidered to be Vwithin the purview and scope of the invention andVappended claims.

We claim:

1. An airborne electromagnetic device adapted for detecting andmeasuring an ore body comprising, in comination with an laircraft and abird towed by the aircraft,

a pair of energizing coils in the aircraft in'mutually or,- thogonalrelationship, a pair of power sources of different frequencies, means toenergize said coils from said power sources to radiate electromagneticelds of diierent frequencies, a pair of receiving coils mounted in saidVbird in mutually perpendicular relationship, first frequencydiscrminating means connected to one of said receiving coils and tunedto one of said frequencies, second frequency discriminating meansconnected to the other of said receiving coils and tuned to the other ofsaid frequencies, means for feeding the responses of said two frequencydiscriminating means to said aircraft, mezins mounted in Vsaid aircraftoperatiye to receive and compare said responses from the two frequencydiscriminating means, and electro-optical means fcr Vmaintaining asubstantially constant positional relationship between said aircraft andsaid bird.

2. An airborne electromagnetic apparatus in accordance with claim lwherein said electro-optical means comprises a source of light locatedat said aircraft and further cornprises -a light-responsive system insaid bird responsive to the light fromsaid source for controlling thepositional relationship of said bird to said aircraft.

3. An airborne electromagnetic apparatus in accordance with claimZwherein said light source comprises a source of modulated light.

4. An airborne electromagnetic apparatus in accordance with claim 3wherein said light-responsive system in said bird comprises a pyramidalprism and a plurality of light.

sensitive devices associated with said pyramidal prism and positioned toreceive equal quantities of light from the pyramidal prism when the birdis in a desired positional alignment withrespect Vto the light from saidsource.

5. An airborne electromagnetic apparatus in accordance with claim 4wherein said light-responsive system furtherV .detecting and measuringan ore body comprising, in combination with an aircraft and a rst birdand a second bird towed by the aircraft, an energizing coil arrangementincluding at least one lenergizing coil mounted Vin the first of saidbirds to radiateat least one electromagnetic jield, a detector coilsystemY including at least one detector coil mounted in said secondbird, means for feeding'the `responses from said detector coil system todetecting and indicating means, and-electro-optical apparatus mounted insaid birds for maintaining a substantially constant positionalrelationship between said birds.

7. An airborne electromagnetic apparatus in accordance with claim 6wherein said electro-optical apparatus comprises a source of lightlocated at said aircraft and further comprises alight-responsive systeminV each of said birds responsive to the light from said source forcontrolling the positional alignment of said birds relative to eachother.

8. An airborne electromagnetic apparatus in accordance with claim 7wherein said light source comprises a source of modulated light. Y Y

9. An yairborne electromagnetic apparatus-in accordance with claim 8wherein said light-responsive system in each of said birds comprises apyramidal prism and a plurality of light sensitive devices associatedwith said pyramidal prism and positioned to receive equal quantities oflight from the pyramidal prism when the birds are in a desiredpositional alignment with respect to the light from said source.

l0. An airborne electromagnetic apparatus in accordance with claim 9wherein said light-responsive system in each of said birds furtherincludes filter means in circuit with each of said light-sensitivedevices for accepting'signals due to energization of the light-sensitivedevice by modulated light from said source of lightV and rejectingsignals due to energization of the light sensitive device by otherlight.

1l. An airborne electromagnetic system adapted for detecting andmeasuring a mineral body comprising two airborne vehicles consisting ofan aircraft and a bird towed by the aircraft, a first coil arrangementincluding at least one coil carried by said aircraft and a second coilarrangement including at least one coil in said bird, means connected toone of said coil arrangements to radiate at least one electromagneticeld and means connected to the other of said coil `arrangements forreceiving and detecting the responses due to the at least oneelectromagnetic field radiated by the one coil arrangemen-t, meansconnected to said receiving and detecting means for indicatingvariations inV the Vreceived and detectcd signals and electro-opticalmeans for maintaining a relatively constant positional relationshipbetween said first and said second coil arrangements.

12. An airborne electromagnetic system in accordance with claim 1lwherein said electro-optical means comprises a source of light locatedat one of said airborne vehicles and means for directing a beam of lightfrom the source in the direction of said other of the airborne vehiclesand further comprises electro-optical apparatus in said other of theairborne vehicles responsive to said beam of light to maintain asubstantially constant positional relationship of the bird withrespectfto said beam of light.

i3. An airborne electromagnetic system in accordance with claim 12whereinsaidrlight source includes means for modulating said light beamand wherein said electrooptical apparatus includes meansrresponsive onlyto the modulated light.

14. VAn airborne electromagnetic system inaccordance with claim 13wherein said electro-optical apparatus includes a pyrarnidal prism anda'plurality of photo cells associated with said prism to receive equalamounts of light when the airborney vehicle containing saidelectrooptical apparatus is positioned correctly in relation to saidbeam of light.

15. An airborne electromagnetic system in accordance with claim 14wherein said electro-optical apparatus further includes means fordetecting diderences in the light received in said photo cells andfurther includes control Y means for controlling the position relativeto said light beam of Vthe airborne vehicle containing saidelectrooptical apparatus. Y Y Y Q16, An yairborne electromagnetic systemin accordance with claim 15 wherein two pairs of photo cells areprovided and wherein said means for `detecting variations in the lightreceived from said photo cells includes means connected to pairs of saidphoto cells for detecting the difference in light signal received by onepair of said photo cells and for detecting the difference in lightreceived by the other of said pairs of photo cells.

17. An airborne electromagnetic apparatus as defined in claim ll inwhich Ithe coil arrangement carried by the aircraft is in a housing ofelectrically non-conductive material extending -from the rear of theaircraft.

18. An airborne electromagnetic apparatus as defined in claim 16 inwhich the coil arrangement carried by the aircraft is in a housing ofelectrically non-conductive material extending from the rear of theaircraft.

19. An airborne electromagnetic apparatus as defined in claim l1 inwhich the means to radiate at least one electromagnetic field isconnected to the coil arrangement mounted in the bird.

20. An airborne electromagnetic appara-tus as defined in claim 16 inwhich the means to radiate at least one electromagnetic field isconnected to the coil arrangement mounted in the bird.

2l. An apparatus for controlling the positional relationship betweenairborne craft comprising electro-optical means carried by each of saidairborne craft for maintaining a substantially constant positionalrelationship between the airborne craft.

`2,2. An apparatus in accordance with claim 21 wherein theelectro-optical means in one of said airborne craft comprises a sourceof light and means for directing a beam of light from the source in thedirection of another airborne craft and wherein the electro-opticalmeans in the other airborne craft includes electro-optical apparatusresponsive to said beam of light to maintain a substantially constantpositional relationship between the two craft.

23. An airborne apparatus in accordance with claim 21 wherein anairborne vehicle moving along with other airborne craft in fir'ghtcarries means for directing a separate beam of light in the direction ofeach of said other airborne craft and the electro-optical means in eachof said other airborne craft comprises electro-optical apparatusresponsive to a separate one of said beams of light to maintain asubstantially constant positional relationship between said airbornecraft.

24. An airborne apparatus in accordance with claim 22 wherein said meansfor directing light includes means for modulating said light and whereinsaid electro-optical apparatus includes means responsive only to themodulated light.

25. An airborne apparatus in accordance with claim 22 wherein saidelectro-optical apparatus includes a pyramidal prism and a plurality oflight-sensitive devices associated with said prism to receive equalamounts of light when the airborne vehicle containing the electroopticalapparatus is positioned correctly in relation to its beam of light.

26. An airborne apparatus in accordance with claim 24 wherein saidelectro-optical apparatus includes a pyramidal prism and a plurality oflight-sensitive devices associated with said prism to receive equalamounts of light when the airborne vehicle containing the electroopticalapparatus is positioned correctly in relation to its beam of light.

27. An airborne electromagnetic apparatus adapted for detecting andmeasuring an ore body comprising an aircraft and a bird towed by theaircraft, a receiving coil system mounted in the bird and including atleast one receiving coil in the bird, an amplifier connected to eachreceiving coil, a modulator in the bird connected to the ampliliers forimpressing on an electrical carrier wave the signals received from theamplifiers, conductor means for transmitting the modulated carrier wavefrom the bird to the aircraft, `detecting means in the aircraft todetect and separate from the carrier wave the signals from theamplifiers and means connected to the detecting means for comparingvariations in the received and detected signals, and electro-opticalmeans for maintaining a relatively constant positional relationshipbetween the aircraft and the bird.

28. A method for controlling the positional relationship betweenairborne craft comprising providing a light source moving along insubstantially fixed relation with a rst airborne craft, directing a beamof light from the moving light source in the direction of a secondairborne craft whose position relative to the first airborne craft is tobe controlled so that light from the beam falls equally on a surface atthe second airborne craft when the two craft are properly aligned anddetecting changes in the position of the light from the beam yfalling onsaid surface as an indication of misorientation of the two craft, andutilizing this misorientation to maintain a substantially constantpositional relationship between the airborne craft.

29. An airborne electromagnetic apparatus in accordwith claim 6 whereinsaid electro-optical apparatus comprises a source of light located atone of said birds and a light-responsive system located at the otherbird and responsive to the light from said source for controlling thepositional alignment of said birds relative to each other.

30. An airborne electromagnetic apparatus in accordance with claim 29wherein said electro-optical apparatus comprises another source of lightlocated at said other bird and another light-responsive system locatedat said one bird and responsive to the light from said other source.

31. An airborne electromagnetic apparatus in accordance with claim 6,there being a source of light located at said aircraft, saidelectro-optical apparatus in said birds comprising a light-responsivesystem located at one of said birds responsive to the light from saidsource, another source of light located at said one bird, and anotherlight-responsive system located at the other bird responsive to thelight from said other source.

References Cited n the file of this patent UNITED STATES PATENTS2,623,924 Cartier Dec. 30, 1952 2,642,477 Puranen et al June 16, 19532,741,736 Puranen et al Apr. 10, 1956 2,794,949 Hedstrom et al. June 4,1957

